Recently, with the development of mobile devices such as smart phones, tablet PCs, and the like, substrates for displays have been required to become thin and slim. Display windows or front panels of such mobile devices are generally made of glass or reinforced glass both of which have excellent mechanical properties. However, the glass and reinforced glass are heavy, which leads to a heavy mobile device and easy breakage by an external impact. Further, they are not suitable for use in flexible or foldable display devices due to their low flexibility.
As an alternative to glass, plastic resins have been researched. The plastic resin is lightweight and less likely to be broken, and has flexibility, which makes it more suitable for lightweight and flexible mobile devices. For example, polyethylene terephthalate (PET), polyethersulfone (PES), polyethylene naphthalate (PEN), polyacrylate (PAR), polycarbonate (PC), polyimide (PI), and the like are used. However, substrates using these plastic resins have a problem that hardness and scratch resistance are insufficient as compared with glass materials. Accordingly, attempts have been made to improve hardness and abrasion resistance by coating a resin composition on a plastic resin substrate to form a hard coating layer.
For example, hard coating on substrates of foldable displays mainly focuses on achieving surface hardness and scratch resistance to ensure conventional glass-like mechanical properties. However, when it is applied to a film, flexibility may be deteriorated, and thus the hard coating film may be cracked. In addition, a thin acrylic hard coating film having high hardness and flexibility has been proposed, but it has high shrinkage during curing, thereby causing severe curling and imposing limitations on impact resistance.
In addition, among the hard coating on substrates of foldable displays, an outer folding type in which the hard coating film is folded outward applies higher stress to the hard coating film as compared with an inner folding type, and thus cracks are likely to occur. As a result, conventional hard coating compositions have a limitation in providing outer folding having a low curvature of 10 mm or less in diameter.